Electric switching device

ABSTRACT

An electric switching device for fast closing of a high current in a power network comprising a first electrode ( 1 ), a second electrode ( 2 ), a movable contact element ( 3 ) closing said first and second electrodes, and an operating device. The operating device comprises a helically wound first coil ( 6 ) secured to the first electrode. The movable contact element comprises a flange ( 4 ) making contact with the coil. A current pulse flowing through the coil forms a repulsive force between the coil and the flange, said repulsive force throwing the movable contact element to the second electrode and completing the closing operation.

TECHNICAL FIELD

The present invention relates to an electric switching device for highcurrent in an electrical power network. More particularly, the inventionrelates to a so-called high-speed circuit closer which is adapted toachieve a fast mechanical electric short circuit of at least one phasein a multi-phase network. The switching device is preferably intended tobe used as arc eliminator in cubicle-enclosed switchgear in distributionnetworks and in transmission networks. Thus, the invention comprisesboth low, medium and high voltage. In accordance with the IEC standard,medium voltage means 1-72.5 kV whereas high voltage is >72,5 kV.

BACKGROUND ART

In arcs, caused by large fault currents, very large amounts of energyare released in the form of heat and radiation. In, for example,enclosed switchgear, with its limited space, these amounts of energygive rise to increases in pressure which may blast the enclosure. Suchswitchgear must therefore be equipped with space-demanding reliefopenings through which the heated gases are given a possibility to flowout. Further, the high arc temperatures cause the material in conductorsand in switching equipment to melt and even evaporate. Burnable organicmaterial may also be ignited when subjected to the high temperature andintense radiation of the arc. By decomposition of air (NOx) andevaporation of metals, the arc gives rise to poisonous gases. It is,therefore, common for such switchgear to be provided with devices forpressure relief in the form of evacuating channels, automaticallyopenable hatches, etc. This means that such switchgear will be bulky andcostly.

In industry, there has long been a great need to primarily prevent arcsfrom arising. In the second place, there has been a need to minimize thearc duration. In this way, material damage as a result of the heat andthe pressure increase, built up during the duration of the arc, can bereduced. Also, the risk of personal injury and poisoning is reduced.

In case of a duration of the arc of about 30 ms, a switch-gear unit maybe completely blown out. The pressure wave caused by the arc usuallyreaches its maximum even after 10-25 ms. To reduce the material damage,there is thus a need to limit the arc duration to about 10 ms. To reducethe risk of personal injury, still shorter arc durations should be aimedat. However, known circuit breakers have a considerably longerbreak-time. One common way of limiting the duration of the arc is,therefore, to rapidly switch the fault current to ground before theactual breaking operation.

A common problem in connection with a closer is that burns arise on thecontact surfaces. Immediately before the switching elements come intocontact with each other, arcs arise which partially melt the metal inthe contact elements. This deteriorates the contact and causesdevelopment of heat. Known closers must therefore be overhauled at shortintervals.

From, for example, DE-A-2623816, it is previously known to use, ingas-insulated, metal-enclosed switchgear, a fast grounding switch toextinguish a short-circuit arc between a high-voltage conductor and thegrounded enclosure, to thus eliminate the risk of a dangerousoverpressure building up. The grounding switch described in thepublication is of single-phase design and is operated by a built-inexplosive charge, the ignition of which is initiated by a sensoractuated by the arc. One disadvantage with such a grounding switch isthat it must undergo a general overhaul or be replaced after one singleoperation. A further disadvantage is the handling and storage ofexplosives during such an overhaul.

From WO 97/45851, a high-speed circuit closer is previously known which,with a torsion-sprung contact device, brings a knife-shaped contact partinto contact with a fork-shaped contact part. Both contact parts arehoused in a container with insulating gas. The task of the knownhigh-speed circuit closer is to achieve a rapid closing and preventburns from arising on the contact parts. One disadvantage with thishigh-speed circuit closer is that a relatively large number of parts areincluded in the actual movement. The composition of the breaker is alsorelatively complicated. For optimum function, the torsion spring has tobe adjusted carefully. A further disadvantage is that the closer onlyfunctions as a closer. Thus, it requires manual return to the insulatingposition. The spring must then be tensioned to be put into operationalcondition again.

SUMMARY OF THE INVENTION

The object of the present invention is to suggest ways and means ofachieving a fast electric switching device, the break-time or make-timeof which is less than about 5 ms. In its function as a grounding switch,a so-called arc eliminator, it shall prevent the occurrence of arcs atthe moment of contact so as to avoid damage to the contact elements. Itshall effectively brake the movable contact system during a closingoperation. The switching device shall be able to carry out severalclosing and opening operations in rapid succession, also in the reverseorder. The switching device shall manage both a high voltage and a highcurrent. It shall have a simple and compact design which makes possibleinstallation in conventional air-insulated switchgear without theabove-mentioned disadvantages which are associated with prior artdesigns.

These objects are achieved according to the invention by an electricswitching device according to the characteristic features described inthe characterizing portions of the independent claims 1 and 6 and by amethod according to the characteristic features described in thecharacterizing portions of the independent method claims 7 and 8.Advantageous embodiments are described in the characterizing portions ofthe dependent claims.

In high-voltage switchgear, grounding switches of two kinds are used,namely, working grounding switches and high-speed grounding switches.From a first aspect of the invention, the switching device is adapted toconstitute a grounding switch of the latter kind. Such a fast groundingswitch, a high-speed grounding switch, shall manage to ground thehigh-voltage parts also when these are energized and in case of largefault currents. In such a switching case, the contacts are subjected tofull short-circuit current. In order thus to limit the contact burn-offand other function-reducing effects which are caused by the arcs whichare usually created during the closing operation, bouncing movementsbetween the contacts must be limited or completely eliminated. Thebouncing movements are primarily dependent on the speed at which thecontact parts butt against each other. The amplitude of the bouncingmovement thus increases with the relative final speed between thecontact parts. An important task of the high-speed circuit closer istherefore to achieve a low speed of the contacts at the moment ofclosing and an ability to damp the kinetic energy of the contacts.

From a second aspect of the invention, the electric switching device isadapted to constitute a circuit breaker. Usually, a circuit breaker mustbe able to handle the occurrence of an arc between the contacts. To thisend, a conventional circuit breaker is equipped with means forextinguishing an arc thus arisen. However, the switching deviceaccording to the invention is so fast that no significant amount ofenergy has time to build up in the arc. The arc therefore expires byrapid separation of the contacts during zero crossing in the current.

The switching device according to the invention is arranged with a firstcontact part, a second contact part and a movable contact partdisplaceable therebetween. In the open position, the movable contactpart rests at the first contact part. In the closed position, themovable contact part rests in contact with both the first contact partand the second contact part, whereby the current is passed through themovable contact part. In a first stage of the closing or openingmovement, a rapid acceleration is imparted to the movable contact part.In a subsequent stage, the movable contact part moves at constant speed.In a conventional arc eliminator, a movable contact part usually movesunder continuous acceleration during the whole movement, whereby a highspeed of the movable contact part is achieved at the moment of closing.In a switching device according to the invention, where the movement isdivided into an acceleration phase and a phase with a constant speed, alower speed of the movable contact is obtained, for the same make-time,at the moment of closing.

The accelerating movement of the movable contact part is achieved with aso-called Thomson coil. When a current pulse traverses a flat-woundhelical coil, the coil forms a variable magnetic field. In an adjacentmetallic object, eddy currents are formed which, in turn, create avariable magnetic field directed in the opposite direction. For largecurrent pulses, a strong repulsive force is formed between the coil andthe adjacent metallic object. However, the force decreases rapidly withthe distance. According to the invention, the movable contact part isdesigned as a sleeve of metal with a flange arranged at one end. Thesleeve surrounds the first contact part and is arranged, in its openposition of rest, such that the flange makes contact with a helical,flat coil arranged around the first contact part. When a current pulsetraverses the coil, a strong repulsive force is thus formed between thecoil and the flange, which force pushes away the sleeve against thesecond contact part.

For the breaking movement of the switching device, a Thomson coil isarranged in a similar manner. Secured to a housing of insulatingmaterial, surrounding the contacts, a helically wound flat coil isarranged. Spring-loaded against this coil, a metallic ring surroundingthe sleeve is arranged. When a current pulse traverses the coil, arepulsive force arises which pushes the ring in a direction away fromthe second contact part. The ring and the flange of the sleeve arearranged such that the ring hits the flange. Upon this contact, the ringtransfers its kinetic energy to the flanged sleeve, causing the sleeveto resume its open position of rest.

Between two poles with different potential, an electric field arises.The appearance of this field is largely dependent on the shape of theelectrodes. If the electrodes are pointed or contain edges orirregularities, concentrations in the electric field arise. Suchconcentrations in the field may give rise to partial discharges. Thesedischarges may, in turn, be the embryo of arcs. The switching deviceaccording to the invention is arranged with round electrodes, wherebyconcentrations in the electric field are avoided. As a result thereof,it has been possible to reduce the distance between the electrodes incase of full insulation. A smaller distance gives less energyrequirement for moving a contact element which brings about contactbetween the electrodes. Due to the smaller distance it also takes lesstime to bring about contact. At high voltages, the contacts are arrangedenclosed in a container filled with insulating gas, the insulating gasbeing utilized to further reduce the distance between the contact parts.

When two contacts approach each other, a small arc arises at the momentof contact. This is dependent on the current but also on the bouncingmovement which arises between the contacts during the impact. The energywhich causes these movements is dependent on the speed squared. Thismeans that a reduction of the speed is exceedingly favourable forreducing the occurrence of bouncing movements. In case of a slidingcontact, where the contact parts are moving in parallel with the contactsurface, bouncing effects also occur in that a transverse force isimparted, upon impact, to the contact parts, which force sets thecontact parts in oscillation. The oscillation causes the contact partsto alternately be in contact with each other and alternately be at adistance from each other. During the short time during which the contactparts are separated from each other, an arc arises which causes damageto the contact surfaces.

According to the invention, the harmful arcs are eliminated by means ofa plurality of spring fingers. These are made of a conducting materialwith a high mechanical yield point. The spring fingers are placed incontact with the second contact part and are adapted, upon oscillation,to exhibit a high mechanical resonant frequency. When the movablecontact part is brought to rapidly slide against the second contactpart, it first hits the contact fingers, which are thrown sideways. Bythis force, the spring fingers are set into oscillation such that abouncing movement arises. The spring fingers are dimensioned to obtain ahigh resonant frequency. On its way, the movable contact part knocksagainst a plurality of fingers, which are all brought into oscillation.The spring fingers are struck at different times and have differentresonant frequency. This means that the phase difference between theoscillation of the different fingers will be random. Since theoscillation frequency is high, some finger will always be in contactwith the movable contact part. This means that arcs do not arise andwhen the movable contact part has assumed the closed position, also thevibration of the contact fingers has decreased, enabling the groundingswitch to carry the current,

As contact-making member between each of the first and the secondcontact part and the movable contact part, one or a plurality of tapesof helically wound wire are used. In conventional switching devices,this type of contact member occurs at the corresponding contact betweenthe first contact part and the movable contact part. However, it is notknown to arrange such a contact member for providing contact betweencorrespondingly the second contact part and the movable contact part.Thus, conventional switching devices exhibit a fixed contact part,corresponding to the second contact part, which has contact fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by description of anembodiment with reference to the accompanying drawing, wherein

FIG. 1 shows an explanatory sketch of a switching device with two fixedand one movable contact part according to the invention, and

FIG. 2 shows an advantageous embodiment of the switching device,comprising a device for achieving breaking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A switching device according to the invention includes a first contactpart 1, a second contact part 2 and a movable contact part 3. Themovable contact part is formed as a sleeve. The sleeve surrounds thefirst contact part and is formed with a flange 4. Between the first andsecond contact parts, respectively, and the movable contact part,contact members 5 are arranged. In the embodiment shown, these contactmembers are designed in the form of tapes of helically wound wire. Thetapes surround the first and second contact part, respectively, and arearranged in grooves in the respective contact part. The helically woundshape permits the tapes to make contact with a resilient force with therespective contact part. The contact members are adapted to allow themovable contact part a longitudinal movement with a retained low-ohmiccontact with each of the first and the second contact part,respectively.

A first helical coil 6, a so-called Thomson coil, is arranged adjacentto the flange 4. In the figure, the helical coil is arranged immediatelybelow the flange, the flange being adapted to make contact with thecoil. When a current pulse flows through the coil from a current source(not shown), a variable magnetic field arises, which induces eddycurrents in the flange 4. The eddy currents, in turn, give rise to theformation of a magnetic field directed opposite to the first magneticfield. This gives rise to a strong repulsive force which throws away theflanged sleeve 3 against the second contact part.

A plurality of arc fingers 7 of spring steel are arranged around thesecond contact part. These arc fingers are clamped into contact with thesecond contact part, in the upper part of the figure, and have theirfree ends, the fingertips, directed obliquely to the first contact part.The arc fingers are adapted to exhibit a deflection with a high resonantfrequency. When the fingers are hit by the forward-moving movablecontact part, a vibrating movement thereof arises. The fingertips thenbounce against the movable contact part. Each time the fingertip leavesthe surface of the movable contact part, a small arc arises. However,since a plurality of fingers are arranged around the second contact partand all lie in different phases and have different resonant frequencies,at all time always some fingertip is in contact with the movable contactpart. This causes these arcs to be eliminated.

FIG. 2 shows an advantageous embodiment of a switching device accordingto the invention. The embodiment comprises all the previously mentionedparts which are indicated by the same reference numerals. In the exampleshown, the contacts are arranged in an enclosure of an insulatingmaterial. The enclosure, which suitably is filled with a protective gas,consists, according to the example shown, of a cylindrical wall 8, a toppart 9 and a bottom part 10.

Upon a closing operation, the movable contact part is thrown against thesecond contact part such that, in the closing position, this contactpart brings about contact with the first contact part and the secondcontact part. This is done with the aid of the contact members 5. In theclosing position, the flange 4 makes contact with a hammer ring 12,which in turn makes contact with a second helical coil 11. The hammerring is movable along the extent of the movable contact part and ismaintained in contact with the second helical coil by the force from aspring 13. The second helical coil is secured to the bottom part 10.When a current pulse from a current source (not shown) flows through thesecond helical coil, a variable magnetic field arises, which induceseddy currents in the hammer ring 12. The eddy currents, in turn, giverise to the formation of a magnetic field directed opposite to the firstmagnetic field. This gives rise to a strong repulsive force which throwsaway the hammer ring against the flange of the movable contact part. Thekinetic energy thus established is transferred to the flange of themovable contact part which is thrown to its open position of rest. If,for some reason, the movable contact part should not have completelyreached the closed position, the hammer ring will transfer the kineticenergy to the movable contact through an impulse. The solution shown forthe breaking function is thus independent of whether the precedingclosing operation has been complete.

A shield ring 14 is arranged on a level with that end of the firstcontact end which faces the second contact part. In a corresponding way,a shield ring 15 is arranged on a level with that end of the secondcontact part which faces the first contact part. The two shield ringsare adapted to distribute the electric field occurring between the firstcontact part and the second contact part, such that no fieldconcentrations arise.

What is claimed is:
 1. An electric switching device for fast closing ofa high current in a power network comprising a first plug-shapedelectrode, a second plug-shaped electrode, a movable contact element forclosing said first and second electrodes, and an operating device,wherein the contact element is cylinder-shaped and surrounds the firstelectrode in an open position of the switching device, the operatingdevice comprises a helically wound first coil secured to the firstelectrode, and the contact element comprises a flange making contactwith the coil, a current pulse flowing through the coil forming arepulsive force between the coil and the flange, said repulsive forcethrowing the movable contact element to the second electrode, whereby,in a closed position of the switching device, the contact elementsurrounds both the first electrode and the second electrode.
 2. Aswitching device according to claim 1, wherein between the secondelectrode and the movable contact element in the closed position, thereare arranged contact members comprising helically wound, conductingwire.
 3. A switching device according to claim 1, wherein a plurality ofarc fingers of spring steel are arranged at the second electrode, saidarc fingers being adapted to prevent the occurrence of arcs between thesecond electrode and the movable contact element.
 4. A switching deviceaccording to claim 1, wherein a first shield is arranged at the firstelectrode and a second shield is arranged at the second electrode, thefirst and second shields being adapted to distribute between them theelectric field.
 5. A switching device according to claim 1, wherein ahammer ring, displaceable along a longitudinal direction of the movablecontact element, is arranged making contact with a secured secondhelical coil, a current pulse flowing through the coil forming arepulsive force between the coil and the hammer ring, which repulsiveforce throws the hammer ring against the flange of the movable contactelement, whereby kinetic energy is obtained and is transferred to themovable contact element which returns to the original position.
 6. Anelectric switching device for fast breaking of a high current in a powernetwork comprising a first plug-shaped electrode, a second plug-shapedelectrode, a movable contact element for closing said first and secondelectrodes, and an operating device, wherein the movable contact elementis cylinder-shaped and surrounds both the first electrode and the secondelectrode in a closed position of the switching device, said operatingdevice comprises a helically wound coil and a hammer ring making contactwith the coil and being displaceable along the movable contact element,and the movable contact element comprises a flange for receiving contactwith the hammer ring, a current pulse flowing through the coil forming arepulsive force between the coil and the hammer ring, said repulsiveforce throwing the hammer ring against the movable contact element,whereby transferred kinetic energy causes the contact element to assumean open position of the switching device.
 7. A method for fast closingof a high current in a power network comprising a second plug-shapedelectrode which is brought into contact with a first plug-shapedelectrode by a movable contact element, wherein the movable contactelement is cylinder-shaped, the contact element in an open position ofthe switching device is adapted to surround the first electrode, thecontact element comprises a flange which is brought to make contact witha helically wound coil secured to the first electrode, and the coil isbrought to be traversed by a current pulse such that a repulsive forcebetween the coil and the flange is formed, whereby the movable contactelement is thrown to the second electrode, whereupon both the firstelectrode and the second electrode are surrounded by the contactelement.
 8. A method for fast breaking of a high current in a powernetwork wherein a movable contact element, which brings about contactbetween a first plug-shaped electrode and a second plug-shapedelectrode, is brought to he displaced from the second electrode to thefirst electrode such that breaking arises, wherein the movable contactelement is arranged cylinder-shaped and surrounding both the first andthe second electrode in a closed position of the switching device, thecontact element comprises a flange, a hammer ring, which is displaceablealong the movable contact element, is adapted to make contact with ahelically wound coil, and the coil is brought to be traversed by acurrent pulse, whereby a repulsive force between the coil and the hammerring is formed, whereby the hammer ring is thrown against the flange ofthe movable contact element, whereby a kinetic energy is transferred tothe contact element which brings the contact element to assume an openposition of the switch device.